28o BIOLOGICAL LECTURES. 



remind us of the following passage in the last chapter of 

 Darwin's Descent of Man ? " I am aware that the conclusions 

 arrived at in this work will be denounced by some as highly 

 irreligious ; but he who denounces them is bound to shew why 

 it is more irreligious to explain the origin of man as a distinct 

 species by descent from some lower form, through the laws of 

 variation and natural selection, than to explain the birth of the 

 individual through the laws of ordinary reproduction. The 

 birth both of the species and of the individual are equally parts 

 of that grand sequence of events, which our minds refuse to 

 accept as the result of blind chance. The understanding 

 revolts at such a conclusion, whether or not we are able to 

 believe that every slight variation of structure, — the union of 

 each pair in marriage, — the dissemination of each seed, — and 

 other such events, have all been ordained for some special 

 purpose." 



Both Wolff and Darwin left their theories unfinished. They 

 maintained a transformation of simpler into more complex mat- 

 ter, but they did not succeed in demonstrating how this trans- 

 formation is accomplished. I have already quoted Wolff's con- 

 fession of ignorance of the way in which epigenetic develop- 

 ment is brought about. The doubts entertained by Darwin and 

 his successors concerning the adequacy of natural selection 

 as a complete explanation of descent are familiar to us all. 

 The absolute completeness of the old emboitement and special 

 creation hypotheses doomed them to a speedy death. Wolff's 

 and Darwin's hypotheses have lived because they represented 

 only parts of a great truth. On this account, also, they have 

 supplied and will continue to supply powerful incentives to 

 investigation. 



Both the theory of epigenesis and the modern theory of 

 descent are manifestly imbued with the old Heraclitean and 

 Aristotelean conception of heterogeneity arising from homo- 

 geneity in a continual flux of events. We have come to 

 regard this as the essence of evolution. We still repeat Her- 

 bert Spencer's definition ^ : " Evolution is an integration of 



1 Spencer, Herbert. First Principles. New York, Appleton & Co., 1886. 

 p. 396. 



